This invention is in the field of digital audio amplifiers, and is more specifically directed to noise reduction in pulse-width-modulation type class D audio power amplifiers.
In recent years, digital signal processing techniques have become prevalent in many electronic systems. Tremendous increases in the switching speed of digital circuits have enabled digital signal processing to replace, in large part, analog circuits in many applications. For example, the sampling rates of modern digital signal processing are sufficiently fast that digital techniques have become widely implemented in audio electronic applications.
Digital techniques for audio signal processing now extend to the driving of the audio output amplifiers. A new class of amplifier circuits has now become popular in many audio applications, namely “class D” amplifiers. Class D amplifiers drive a complementary output signal that is digital in nature, with the output voltage swinging fully from “rail-to-rail” at a duty cycle that varies with the audio information. Complementary metal-oxide-semiconductor (CMOS) output drive transistors are thus suitable for class D amplifiers, as such devices are capable of high, full-rail, switching rates such as desired for digital applications. As known in the art, CMOS drivers conduct extremely low DC current, and their resulting efficiency is especially beneficial in portable and automotive audio applications, as well as in small form factor systems such as flat-panel LCD and plasma televisions, and DVD receivers. The ability to realize the audio output amplifier in CMOS has also enabled integration of an audio output amplifier with other circuitry in the audio system, further improving efficiency and also reducing manufacturing cost of the system. This integration also provides performance benefits resulting from close device matching between the output devices and the upstream circuits, and from reduced signal attenuation.
By way of further background, a particular problem in class D audio amplifiers is presented by the transient events of muting and un-muting of the audio system. As is fundamental in the art, a steady-state square wave time-domain signal (corresponding to a 50% duty cycle PWM signal) transforms into the frequency domain as discrete frequency components at the fundamental “carrier” frequency and its harmonics. It has been observed that if the PWM signal is abruptly gated on or off or otherwise abruptly changes its duty cycle, however, significant energy is present in sidebands to the carrier frequency and its harmonics. And even if the fundamental frequency is relatively high, the abrupt gating on or off of the PWM signal can result in sidebands with significant energy that extend into audible frequencies, which manifest as audible “clicks” or “pops”. In audio systems, this gating on and off of the PWM output occurs when the user mutes or unmutes the audio output, and at power-up and power-down, in which case the audible clicks and pops are very undesirable.
Known analog techniques for reducing clicks and pops in analog audio amplifiers include smoothing the change in biasing, for example at power-up. However, these smooth biasing changes cannot be directly applied in class D amplifiers, because these amplifiers operate by way of PWM switching of the output transistors. According to another conventional analog approach, clicks and pops are reduced by introducing a switch or relay that disconnects the load during mode changes, thus eliminating transients from appearing at the load; however, the insertion and control of such a switch or relay has proven to be cost-prohibitive, especially in modern systems.
Considering that class D audio amplifiers effectively operate in the digital realm, and also considering that many sources of audio input signals are also digital in nature (e.g., compact discs, MP3 and other digitally compressed music files, satellite radio), many modern audio systems are fully digital, in that they receive digital input signals and produce digital, PWM, class D amplifier output. In these fully digital systems, digital signal processing techniques for suppressing clicks and pops are known.
One digital technique for suppressing clicks and pops relies upon the generation of a specific sequence of PWM signals that are designed to cancel out audible frequencies that result from the starting or stopping of the PWM output, as described in U.S. Pat. No. 6,720,825, assigned to Texas Instruments Incorporated and incorporated herein by this reference. Audible noise reduction by stopping a noise-shaped signal at a favorable time, by monitoring the digital output of a noise shaping filter in a digital audio system, is described in U.S. Patent Application Publication No. US 2004/0017854, which is assigned to Texas Instruments Incorporated and incorporated herein by this reference. Another approach to reduction of clicks and pops involves the insertion of inter-channel delay among multiple channels in a digital audio system, as described in copending application Ser. No. 10/988,268 filed Nov. 12, 2004 entitled “On-the-Fly Introduction of Inter-Channel Delay in a Pulse-Width-Modulation Amplifier”, assigned to Texas Instruments Incorporated and incorporated herein by this reference. In this approach, the inter-channel delay is designed to reduce switching noise between the pulse-width-modulated outputs, reduce cross-talk among the multiple channels, and generally provide significant improvement in system performance. These digital techniques have proven valuable in eliminating audible transient effects in digital amplifiers.
The so-called “automute” feature is also important in avoiding unpleasant audible noise, as known in the art. In typical class D digital audio amplifiers, a zero amplitude audio signal is reflected as a 50% duty cycle in the PWM output, which dissipates energy and also generates audible idle noise. According to conventional automuting techniques, the digital output is forced to zero in response to the audio signal having an amplitude below a certain low threshold level for a certain duration. Rapid entry into and exit from the automute state is of course desired for good system performance. However, typical modern audio amplifiers include low frequency filters that, as a result, necessarily have long energy storage times. These energy storage times are longer than the desired response timing for entering the automute state. If the automute state is entered rapidly while energy remains in the filters, however, the resulting transients will generate substantial noise in audible frequencies.